Constitutionally, political parties in Mexico must promote the participation of the people in the democratic life of the country, contribute in the representation of the nation and citizens, and be the access through which citizens can participate in public office, through whatever programs, principles and ideals they postulate.[1] All political parties must be registered before the National Electoral Institute (INE), the institution in charge of organizing and overseeing the federal electoral processes, and must obtain at least 2% of votes in the federal elections to keep their registry. Registered political parties receive public funding for their operation and can also obtain private funding within the limits prescribed by the law. As of 2010 the following political parties are registered before the IFE and all have representatives at the Congress of the Union:

Political parties are allowed to form alliances or coalitions to nominate candidates for any particular election. The coalition must present itself with a particular name and logo. Proportional representation (plurinominal) seats are assigned to the coalition based on the percentage of votes obtained in the elections, and then the coalition reassigns them to the constituent political parties. Once each party in the coalition has been assigned plurinominal seats, they do not necessarily continue to work as a coalition in government.

Throughout the 20th century, PRI had an almost hegemonic power at the state and federal level, which slowly began to recede in the late 1980s. Even though since the 1940s, PAN had won a couple of seats in the Congress, and in 1947 the first presidential municipality (in Quiroga, Michoacán),[2] it wasn't until 1989, that the first non-PRI governor of a state was elected (at Baja California). It was in 1997, that PRI lost its absolute majority at the Congress of the Union, and in 2000 the first non-PRI president was elected since 1929.

Suffrage is universal, free, secret and direct for all Mexican citizens 18 and older, and is compulsory (but not enforced). The identity document in Mexico serves also as the voting card, so all citizens are automatically registered for all elections; that is, no pre-registration is necessary for every election. All elections are direct; that is, no electoral college is constituted for any of the elections at the federal, state or municipal level. Only when an incumbent president is absolutely absent (either through resignation, impeachment or death), the Congress of the Union constitutes itself acts as an electoral college to elect an interim president by absolute majority.

Presidential elections are scheduled every six years, except in the exceptional case of absolute absence of the president. Legislative elections are scheduled every six years for the Senate, to be fully renewed in elections held concurrently with the presidential elections; and every three years for the Chamber of Deputies. Elections are usually held on the first Sunday of July. State governors are also elected every six years, whereas the legislatures are renewed every three years. State elections need not be concurrent with federal elections. Federal elections are organized and supervised by the autonomous public Federal Electoral Institute, whereas state and municipal elections are organized and supervised by electoral institutes constituted by each state of the federation. Elections within the Federal District are also organized by a local electoral institute.

A strongly ingrained concept in Mexican political life is "no reelection." The theory was implemented after Porfirio Díaz managed to monopolize the presidency for over 25 years. Presently, Mexican presidents are limited to a single six-year term, and no one who has held the office even on a caretaker basis is allowed to hold the office again. Deputies and senators are not allowed to immediately succeed themselves.

The presidential elections were the most competitive in the history of the country in which the difference in the ballot count between the winner and the first runner up was less than one percent point, and in which neither candidate got absolute majority in a system in which a second round of voting has not been instituted. Felipe Calderón got the greatest number of votes according to the preliminary computation (PREP) and the ballot recount. Andrés Manuel López Obrador contested the results and demanded a vote-per-vote recount, which was denied by the Federal Electoral Tribunal, based on the argument that inconsistencies could not be proved for all electoral circumscriptions, but order a partial recount of votes of those that did show inconsistencies which represented 9.2% of the total, after which the results were not significantly altered. The Federal Electoral Tribunal declared Felipe Calderón the winner of the elections on September 5, and president elect. He took office on December 1, 2006 and his term ended on November 30, 2012. The current president of Mexico is Enrique Peña Nieto, having taken office on December 1, 2012, his term will end on November 30, 2018.

The concurrent congressional elections were not contested by any party. Both chambers were completely renewed and no party obtained absolute majority. All deputies and senators took office on September 1. First-past-the-post plurality candidates (FPTP) of coalitions represent the parties of which they are members. Proportional representation (PR) seats assigned to coalitions were further reassigned to their constituent parties in whatever manner and number they agreed upon. Parties that formed a coalition for the general elections may continue to work together but they do not form a unified political bloc at the Congress; parliamentary groups are identified by parties and not by coalitions.[3]

The 64 Senate first-past-the-post (FPTP) seats are assigned to the pair of senators of the same party (who run together) that obtain the majority of votes per state and the Federal District. The 32 first minority (FM) seats are assigned to the first runner-up per party and the Federal District. Finally, 32 proportional representation (PR) seats are assigned according to national votes to the party or coalition in relation to the total number of votes obtained nationally. PR seats are assigned to the coalition who then reassigns them to its constituent parties in whatever manner and number they had originally agreed upon, and may or may not work as a bloc in the Senate.

In 1929, all factions and generals of the Mexican Revolution were united into a single party, the National Revolutionary Party (NRP), with the aim of stabilizing the country and ending internal conflicts. During the following administrations, since 1928, many of the revolutionary ideals were put into effect, among them the free distribution of land to peasants and farmers, the nationalization of the oil companies, the birth and rapid growth of the Social Security Institute as well as that of Labor Unions, and the protection of national industries. The party was later renamed the Mexican Revolution Party and finally the Institutional Revolutionary Party. The social institutions created by the party itself provided it with the necessary strength to stay in power. In time, the system gradually became, as some political scientists have labeled it, an "electoral authoritarianism",[4] in that the party resorted to any means necessary, except that of the dissolution of the constitutional and electoral system itself, to remain in power. In fact, Mexico was considered a bastion of continued constitutional government in times where coup d'états and military dictatorships were the norm in Latin America, in that the institutions were renovated electorally, even if only in appearance and with little participation of the opposition parties at the local level.

The first cracks in the system, even though they were merely symbolic, were the 1970s reforms to the electoral system and the composition of the Congress of the Union which for the first time incorporated proportional representation seats allowing opposition parties to obtain seats, though limited in number, in the Chamber of Deputies. As minority parties became involved in the system, they gradually demanded more changes, and a full democratic representation. Even though in the 1960s, a couple (of more than two thousand) municipalities were governed by opposition parties, the first state government to be won by an opposition party was Baja California, in 1989.

The presidential elections held in 1988 marked a watershed in Mexican politics, as they were the first serious threat to the party in power by an opposition candidate: Cuauhtémoc Cárdenas, a defector from the ruling Institutional Revolutionary Party (PRI) and son of former President Lázaro Cárdenas, who was nominated by a broad coalition of leftist parties. He officially received 31.1 percent of the vote, against 50.4 percent for Carlos Salinas de Gortari, the PRI candidate, and 17 percent for Manuel Clouthier of the National Action Party (PAN). It was widely said that Cardenas had won the election, but that the then government-controlled electoral commission had altered the results after the infamous "glitch in the system" (se cayó el sistema, as it was reported). In the concurrent elections, the PRI came within 11 seats of losing the majority of Chamber of Deputies, and opposition parties captured 4 of the 64 Senate seats—the first time that the PRI had failed to hold every seat in the Senate. Capitalizing on the popularity of President Salinas, however, the PRI rebounded in the mid-term congressional elections of 1991, winning 320 seats.

Subsequent changes included the creation of the Federal Electoral Institute in the 1990s and the inclusion of proportional representation and first minority seats in the Senate. The presidential election of 1994 was judged to be the first relatively free election in modern Mexican history. Ernesto Zedillo of the PRI won with 50.2 percent of the vote, against 26.7 percent for Diego Fernández de Cevallos of PAN and 17.1 percent for Cardenas, who this time represented the Party of the Democratic Revolution (PRD). Although the opposition campaign was hurt by the desire of the Mexican electorate for stability, following the assassination of Luis Donaldo Colosio (the intended PRI candidate) and the recent outbreak of hostilities in the state of Chiapas, Zedillo's share of the vote was the lowest official percentage for any PRI presidential candidate up to that time.

Numerous electoral reforms implemented after 1989 aided in the opening of the Mexican political system, and opposition parties made historic gains in elections at all levels. Many of the current electoral concerns have shifted from outright fraud to campaign fairness issues. During 1995-96 the political parties negotiated constitutional amendments to address these issues. Implementing legislation included major points of consensus that had been worked out with the opposition parties. The thrust of the new laws has public financing predominate over private contributions to political parties, tightens procedures for auditing the political parties, and strengthens the authority and independence of electoral institutions. The court system also was given greatly expanded authority to hear civil rights cases on electoral matters brought by individuals or groups. In short, the extensive reform efforts have "leveled the playing field" for the parties.

The 2006 elections saw the PRI fall to third place behind both the PAN and the PRD. Roberto Madrazo, the presidential candidate, polled only 22.3 percent of the vote, and the party ended up with only 121 seats in the Chamber of Deputies, a loss of more than half of what the party had obtained in 2003, and 38 seats in the Senate, a loss of 22 seats. Nevertheless, at the state level, more states are still governed by PRI than by the rest of the parties.